An inverter drive, sometimes referred to simply as an inverter, is a solid state device used to control the speed of a three phase electric motor. As the name implies, an inverter drive operates by receiving AC electrical power, internally converting (or inverting) that power into DC electrical power, and then outputting the DC power in a variably pulsed form to control the speed of an associated three phase motor. Thus, for example, longer DC pulses would provide higher motor speeds, while shorter pulses would provide lower speeds.
Soft starters and solid state starters operate in a manner similar to inverter drives except these types of starters do not incorporate a means for speed control. For the sake of simplicity we will describe and refer to inverter drives in the remaining part of this discussion, but by implication soft starters, solid state starters and other types of starters using solid state components are included and hereinafter are referred to as inverter drives.
Such inverter drives are common in industry and are used in conveyors, fans, cooling towers, extruders, and many other applications. However, as solid state devices, inverter drives are vulnerable to damage from electrical disturbances such as lightning strikes, power surges, low voltages, and other disturbances in the electrical line. When a disturbance of sufficient magnitude occurs, the inverter drive may fail, thus preventing the motor from operating, and stopping the application for which the motor was being used. In addition to the damage or loss caused by the cessation of the application, the damage to the inverter itself may also be costly and/or time-consuming to remedy.
Although certain methods have been attempted for the purpose of isolating inverters during electrical disturbances, these methods involve expensive and unreliable contactors, relays and timers for switching. Moreover, the switching used to isolate the inverter drive may itself cause the damage that was to be avoided. This is due to the possibility that sudden changes in the connectivity of the inverter can cause current surges and spikes within the device, damaging sensitive solid state circuitry.
Various solutions have been developed for preventing stray/surge currents from harming the solid state components of inverter drives, including fuses and circuit breakers, voltage regulators and crowbar circuits. A potential disadvantage of fuses and circuit breakers is that they can take time to respond to circuit faults. The time required for the fuse or circuit breaker to blow out is sometimes long enough enough for inverter drive components to be damaged. It is also possible for a power surge to damage equipment without disturbing the fuse or circuit breaker under some conditions.
Voltage regulators, i.e., circuits that regulate voltage output despite changes in input voltage or output load current demand, are widely used in power supplies for inverter drives, controllers and operating devices in equipment. However, as with fuses and circuit breakers, power source voltage regulators may not activate fast enough to protect inverter drives. While crowbar circuits that quickly form a deliberate short circuit across the power supply lines may eliminate some delay in fuse or breaker activation, these safeguards are also prone to failure in the event of very short, infrequent power line transients.
It will be appreciated that this background description has been created by the inventors to aid the reader, and is not to be taken as a reference to prior art, nor as an indication that any of the indicated problems were themselves appreciated in the art.